java八股文面试[java基础]—

java八股文面试[java基础]—— 重载和重写

Java中，有一个名叫方法签名的东西，它的定义是这样的

Definition: Two of the components of a method declaration comprise the method signature—the method’s name and the parameter types.

大概意思是，方法签名有两部分组成——方法名和参数列表

我上面的函数的方法签名就是 hand(int a,int b) 而简单来说，JVM就是通过方法签名来决定去调用哪一个方法的，而从方法签名的组成部分来说，返回类型并不在方法签名中，所以当一个类中出现方法名参数列表相同，只有返回类型不同时，JVM无法根据方法签名来确定该调用哪一个方法，所以会报错。

参考：https://blog.csdn.net/qq_53574615/article/details/130379554

扩展：

可以通过修改字节码，把函数修改成同名：

构造两个不同名字相同参数的成员函数：

public class OverWrite {
    public String getString(String str) {
        return str;
    }

    public void getString1(String str) {
        return;
    }
}

通过javap -v 命令查看字节码：

D:\Program Files\Java\jdk1.8.0_331\bin>javap.exe -v D:\project\test\out\production\test\OverWrite.class
Classfile /D:/project/test/out/production/test/OverWrite.class
  Last modified 2023-8-17; size 504 bytes
  MD5 checksum 40c07ae9b391609cd8fb00b3d2924ce2
  Compiled from "OverWrite.java"
public class OverWrite
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#19         // java/lang/Object."<init>":()V
   #2 = Class              #20            // OverWrite
   #3 = Class              #21            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               LocalVariableTable
   #9 = Utf8               this
  #10 = Utf8               LOverWrite;
  #11 = Utf8               getString
  #12 = Utf8               (Ljava/lang/String;)Ljava/lang/String;
  #13 = Utf8               str
  #14 = Utf8               Ljava/lang/String;
  #15 = Utf8               getString1
  #16 = Utf8               (Ljava/lang/String;)V
  #17 = Utf8               SourceFile
  #18 = Utf8               OverWrite.java
  #19 = NameAndType        #4:#5          // "<init>":()V
  #20 = Utf8               OverWrite
  #21 = Utf8               java/lang/Object
{
  public OverWrite();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   LOverWrite;

  public java.lang.String getString(java.lang.String);
    descriptor: (Ljava/lang/String;)Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=2
         0: aload_1
         1: areturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       2     0  this   LOverWrite;
            0       2     1   str   Ljava/lang/String;

  public void getString1(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 7: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   LOverWrite;
            0       1     1   str   Ljava/lang/String;
}
SourceFile: "OverWrite.java"

可以看到getString 和 getString1 的两个函数定义